Among the more important factors that limit the performance of high-resolution CRT's, such as those required for high-definition television (HDTV), are the enlargement and distortion of an electron beam spot on the tube screen when the beam is deflected away from the center of the screen. Demands for improved CRT's with higher resolution, smaller high-current spots, and flatter faceplates have led to numerous attempts to reduce spot growth and distortion, primarily by minimizing the effects of deflection defocusing caused by the yoke.
Systems that use a self-convergent yoke and an electrostatic stigmator have been developed to eliminate the vertical overfocusing and flare that degrade high-resolution images. However, these systems produce focused electron beam spots, at full horizontal deflection on the screen, that are about twice as wide as the spot at the screen center and therefore are unacceptable for high-resolution CRT's.
Substantial reductions in electron beam spot width at the screen can be achieved in self-convergent systems by the application of two quadrupole lenses, called quadrupole-doublets. These lenses may be either magnetic or electrostatic. Spot width at the screen periphery can be reduced when the two quadrupole lenses are oriented so that the divergent plane of the first quadrupole lens traversed by the beam, and the convergent plane of the second quadrupole lens traversed by the beam, are in the horizontal plane of each of the tube and yoke.
Electrostatic quadrupole doublets are advantageous because they do not affect beam convergence at the screen in multibeam displays. Beam separation at the deflection plane also is not affected by the electrostatic quadrupole doublets, so there is no change required in either the shadow mask contour or in the mask-to-screen spacing within a tube.
One application of an electrostatic quadrupole doublet to reduce spot growth and achieve equal horizontal and vertical spot magnification is disclosed in U.S. Pat. 5,061,881, issued to Suzuki et al., on Oct. 29, 1991. That patent discloses the use of two astigmatic lens fields, one in the beam-forming region and the other located near the main focus lens, on the side of the main focus lens facing the beam-forming region. However, it has been found that placement of both astigmatic lenses between the cathodes and the main focus lens has only a limited capacity for reducing spot growth. Computer simulations, at low beam currents (0.5 to 1 mA), indicate that reductions of less than 20% in spot width can be achieved with this lens arrangement. At higher beam currents, the ability of this lens arrangement to reduce spot width decreases; and for currents above 2 mA, actually increases spot width. The simulations also indicate that the spot correction provided by the lens arrangement is limited by the spherical aberration of the main focus lens. Therefore, the computer simulations have shown that, if the main focus lens is optimally filled with an electron beam, locating a quadrupole doublet before the main focus lens can only degrade spot size at the screen, and that spot uniformity can only be improved at the expense of increased spot size at the center of the screen.